It has been found that certain abnormal growths, such as certain cancerous tissue and atheromatous plaque, have an affinity for certain photosensitizing agents. Photosensitizing agents are compounds that, when exposed to light, or light of a particular wavelength or wavelengths, create O.sub.2 radicals which react with the target cells. Examples of such agents include texaphyrins, hematoporphyrin, chlorins, and purpurins. In the case of living cells, such as cancer tumors, an appropriate photosensitizing agent is used to create the O.sub.2 radicals which kill the target cells. In other situations, such as when it is desired to destroy atheromatous plaque tissue, an appropriate photosensitizing agent is activated to destroy the plaque by lysis (breaking up) of such plaque; mechanisms other than lysis may also be involved.